1. Technical Field
The present invention relates to a method of driving an electro-optical device that uses an electro-optical material such as liquid crystal, the electro-optical device, and an electronic apparatus.
2. Related Art
Liquid crystal is known as an example of electro-optical materials that have optical characteristics that change depending on electric energy. The transmission factor of liquid crystal changes as a voltage applied changes. The change in the transmission factor occurs due to a change in the orientation state of liquid crystal molecules depending on the voltage applied. As the characteristics of liquid crystal, its orientation state is less liable to return to an original state when a direct-current voltage is applied for a long period of time. In consideration of the above characteristics, an alternating-current driving method is generally used in a liquid crystal display device, which uses liquid crystal as its display medium. In the AC driving, the polarity of a voltage that is applied to liquid crystal elements, which constitute a kind of electro-optical elements, is reversed in an alternating manner.
Such a liquid crystal display device typically includes a plurality of scanning lines, a plurality of data lines, and a plurality of pixels that are provided at areas corresponding to respective intersections of the scanning lines and the data lines. Each of the plurality of pixels includes a liquid crystal element. The liquid crystal element includes a pixel electrode, a counter electrode, and liquid crystal. The liquid crystal is sandwiched between the pixel electrode and the counter electrode. As a method for inverting a voltage that is applied to a liquid crystal element, a technique for reversing the polarity of a data potential, which is applied through a data line, with the potential of a counter electrode (hereinafter referred to as “counter electrode potential”) being fixed is known in the art. In the known technique, the polarity of the data potential is reversed with respect to the counter electrode potential, which is the center of the reversal.
Specifically, in the technical field of such a liquid crystal display device, the following technique is disclosed in, for example, JP-A-2003-114661 as a method that takes the place of a voltage modulation scheme for performing grayscale display. One field is divided into a plurality of sub fields. Either an ON voltage or an OFF voltage is applied to a pixel (liquid crystal element) in each of the sub fields. The percentage of time during which the ON voltage (or the OFF voltage) is applied to the pixel in the field is changed for grayscale display. The grayscale-displaying technique is called as a digital time-division drive scheme. In connection with a liquid crystal display device that uses such a sub field, a technique for performing grayscale display while weighting the time periods of sub fields is disclosed in, for example, JP-A-2008-287063. It is known that the disclosed technique makes it possible to express a larger number of gray scale levels in grayscale display with a smaller number of sub fields by actively utilizing the transient response characteristics of liquid crystal.
However, the related art disclosed in JP-A-2003-114661 and JP-A-2008-287063 has the following problem. In these techniques, a switching element such as a thin film transistor is usually used in order to control the time of application of an ON voltage or an OFF voltage to a pixel or a pixel electrode accurately. That is, the switching of a switching element between an ON state and an OFF state is utilized to control the time of application of a voltage to a pixel. However, it is known that a phenomenon called as pushdown occurs during the switchover of the state of a switching element. Pushdown, which is also called as a “field-through” phenomenon or an “overrun” phenomenon, occurs as follows. For example, an n-channel type transistor is used as the switching element. When the switch state of the transistor changes from an ON state to an OFF state, the voltage level of the drain electrode of the transistor drops due to parasitic capacitance between the gate electrode and the drain electrode thereof. Therefore, the voltage level of the pixel electrode connected to the drain electrode drops. Pushdown is a phenomenon of such potential dropping. If no measure were taken against such a phenomenon, the effective value of a voltage applied to the liquid crystal element during writing in negative polarity would be slightly larger than the effective value of a voltage applied to the liquid crystal element during writing in positive polarity. Consequently, without any measure taken against such a phenomenon, a direct-current component would be generated as a predictable problem. The generation a direct-current component increases the risk of the burn-in of a display screen.
To avoid such a problem, in some of related art, the level of a voltage applied to a counter electrode (counter electrode potential) is preset into a value that can offset a potential variation that will arise due to the pushdown explained above. That is, the voltage level is shifted from the center level between two polarities in anticipation of the generation of a direct-current component, thereby offsetting the effects of pushdown. By this means, it is possible to make the effects of pushdown less serious to some extent. However, such a solution of related art is sometimes not so effective in a practical sense. There are various reasons why the above solution might not be so effective practically. For example, according to the solution of related art, “the effects of pushdown” have to have been determined accurately in advance as a prerequisite for setting the voltage level of the counter electrode at an appropriate value at least in principle. However, it is practically difficult to meet the preconditions. Moreover, it is not supposable that the voltage level of the counter electrode will be changed from time to time depending on some circumstances. This is partially because the changing of the counter electrode potential would have a significant impact on other settings and partially because it is uncertain whether the effects of pushdown could be really offset or not due to the reason described above even if the counter electrode potential were changed. To sum up the matter, the solution of related art has a disadvantage in that its flexibility as a measure for effectively avoiding the adverse effects of pushdown is rather limited.